1. Field of the Invention
This invention relates to an LD (laser diode) module, in particular, a low-cost, small-sized and high-performance LD module which is utilized in optical communication systems.
This application claims the priority of Japanese Patent Application No.2000-147867 filed on May 19, 2000 which is incorporated herein by reference.
2. Description of Related Art
FIG. 1 shows a prior art LD module 1 which is sealed in a vertical metal case. This is one of the most prevalent LD modules at present. The transmitting (LD) module 1 has a metallic package. A circular metal stem 2 has a pole 3 erected near the center. An LD (laser diode) 4 is bonded on a side of the pole 3. A top incidence type PD 5 is mounted with a light receiving surface upward at the center of the stem 2. A cylindrical metal cap 6 is welded on the stem 2 for covering the LD 4 and the PD 5. The cap 6 has a top opening 7 for guiding the LD light outward.
A metallic cylindrical lens holder 8 is welded outside of the cap 6 upon the stem 2 for supporting a lens 9 just above the LD 4. A conical ferrule holder 10 is further welded upon the lens holder 8 for maintaining a ferrule 12 at an axial hole. The ferrule 12 holds an end of an optical fiber 11. The LD 4 emits light from both ends in both directions. The LD light has a wide aperture with wide diverging angles. Front light emitted from the front end is converged and guided by the lens 9 to the end of the fiber 11. The front light is signal light. The ends of the fiber and the ferrule are ground slantingly for prohibiting the light reflected at the end from returning to the LD and inducing instability of the LD oscillation. Such a metal packaged three dimensional LD module is still in prevalent, influential use at present. The metal case can hermetically seal the LD chips or the PD chip completely. The metal package prevents corrosive water or oxygen from invading into the package. The hermetic seal protects the module from oxidization or corrosion of the chips, the patterns or the wires. The metal case brings about a long lifetime and high reliability to the module. The metal package is a standardized one which reduces the part cost. The prior LD module of FIG. 1 is still an excellent one.
Further, prevalence of the optical networks strenuously requires lower-cost and smaller-size optoelectronic modules than the metal packaged modules. An improvement of LD modules is a purpose of the present invention. The Inventors are interested in the structure of LDs (laser diodes). Conventional LD modules have commonplace, cheap LD chips having a uniform emission stripe with a constant width w and a constant thickness d extending in the longitudinal direction (z-direction). The emission stripe means a spatially-restricted, active layer which makes laser light by the stimulated emission induced by an electric current injection. FIG. 2 shows a schematic structure of a conventional LD chip for showing only the active layer (stripe) clearly in a perspective view. This is a simple Fabbry-Perrow laser diode. The LD generates light power by inducing light by the injected current, amplifying the light in-phase by the repetitions of propagation in the stripe and reflections at the ends (mirrors) and leaking a part of the amplified light out of the (half-mirror) end. Some sophisticated LDs have light waveguides with a periodical saw-teeth shape upon the stripes for selecting the wavelength more strictly. The shape of the stripe itself is the uniform stripe same as the cheap one of FIG. 2.
For example, an LD emitting 1.3 xcexcm wavelength light has a 300 xcexcm length, a 300 xcexcm width and a 100 xcexcm thickness. The active layer (emission stripe) of the LD chips has e.g., a 1.2 xcexcm width, a 0.2 xcexcm thickness and a 300 xcexcm length (cavity length). The active layer is otherwise called a xe2x80x9ccavityxe2x80x9d, a xe2x80x9cstripexe2x80x9d, an xe2x80x9cemission stripexe2x80x9d or a xe2x80x9cresonatorxe2x80x9d. All the terms signify the same matter. In the prior LD chip in FIG. 2, the width and the thickness of the active layer are constant in the full length. The length of the stripe determines the wavelength of the emitted light. The width and the thickness determine the aperture of the light. The LD light disperses in a wide angle. The aperture angle of the light emitted from the stripe is about 30 degrees to 40 degrees. The wide beam spread is one of the drawbacks of the current LDs. The LD wide beam dispersion sometimes induces difficulties.
An optical fiber consists of a core and a cladding. The core has a 10 xcexcm diameter in a single-mode fiber for the 1.3 xcexcm band light. The cladding has a 125 xcexcm diameter. The LD active layer (w=1.2 xcexcm, d=0.2 xcexcm) is smaller than the fiber core (10 xcexcm) in section. But almost all of the LD light escapes from the fiber and dissipates in vain due to the wide dispersion (30xc2x0 to 40xc2x0) of the LD light, even if the fiber is brought into direct contact with the LD without gap. A lens, therefore, is indispensable for the coupling of an LD to a fiber.
FIG. 1 denotes the lens 9 of converging the dispersing LD light into the end of the fiber 11 for joining the fiber 11 to the LD 4 on an optimum condition. The LD module 1 utilizes a spherical lens having convex spherical curves on both surfaces as converging optics. Sophisticated LD modules employ aspherical lenses for enhancing the coupling efficiency further. Inexpensive LD modules use a ball lens for facilitating the assembly and reducing the cost. In any cases, the three-dimensional structure LD modules require converging optics.
The solid-structured LD module of FIG. 1 is encapsulated in a metal package of a 5.6 mm outer diameter. The LD 4 is a common cheap chip with a uniform stripe as denoted in FIG. 2. An active layer 14 is shown in a chip 13 in a perspective view. As described before, the inexpensive commonplace LD chips have the uniform active layer having a constant breadth w and a constant thickness d. The uniform stripe forgives the light dispersing in a wide aperture. LD modules require converging optics for coupling the LD to the fiber. The converging optics is a ball lens, a spherical lens or an aspherical lens. Low-power, inexpensive modules adopt ball lenses for joining the LD to the fiber.
A ball lens is inexpensive but has large aberration. The aberration prevents the ball lens from heightening the coupling efficiency up to the maximum. Conventional ball lenses allow the LD driving current of about 30 mA to produce an output power of 0.2 mW to 0.5 mW at another fiber end. Thus, the ball lens convergence optics are mainly employed for low-power, low-cost, low-speed and short-range optical communication networks.
On the contrary, high-power, long-range optical communication networks require aspherical lenses for the convergence optics. Aspherical lenses can suppress all kinds of aberration to low levels. The use of the aspherical lens enables the same 30 mA LD driving current to make a 1 mW to 2 mW output power at another fiber end. The coupling efficiency is enhanced several times by the aspherical lens. Aspherical lenses, however, are expensive. The aspherical lenses are used in mainstream fiber cable lines or equipment in central stations.
The ball lens optics and the aspherical lens optics are chosen by the criterion whether the module should be a low-cost, low-power one or a high-cost, high-power one. One purpose of the present invention is to provide a low-cost, high-power, small-size, high-value added LD module. Another purpose of the present invention is to provide an inexpensive ball-lens converging LD module with as high power as the expensive aspherical lens converging LD module. A further purpose of the present invention is to provide an inexpensive ball lens LD loaded module with higher coupling efficiency than the conventional ball lens loaded LD module.
Preparatory descriptions are required for showing the idea of the present invention. A novel type of laser diode has been proposed for coupling the LD to a fiber without convergence optics. The new LD is called a xe2x80x9cspot-size converted laser diodexe2x80x9d. The new SSC-LD has an advantage of eliminating convergence optics from the coupling between an LD and an optical fiber. The present invention is not directed to an improvement of SSC-LDs but to an improvement of LD modules. But the present invention makes use of the novel SSC-LD. The SSC-LDs are not well known even in the skilled.
The SSC-LD is now described before starting the description of the present invention. In short, the SSC-LD has a tapered end stripe produced by reducing the width or the thickness for enlarging the output beam size. Since the LD enlarges the spot size, the LD is called xe2x80x9cspot size convertedxe2x80x9d LD. In spite of the term xe2x80x9cconvertedxe2x80x9d, the SSC-LD does not reduce the beam size but only increases the beam spot size. There are two kinds of the SSC-LDs. One is a width-narrowing SSC-LD, which is called a xe2x80x9chorizontally-taperedxe2x80x9d SSC-LD. The other is a thickness-reducing SSC-LD, which is named a xe2x80x9cvertically-taperedxe2x80x9d SSC-LD.
FIG. 3 shows a vertically-tapered SSC-LD. An LD chip 13 has an active layer 14 with a reducing thickness d toward the front end. The width w of the stripe is constant along the axial line. A coordinate is defined to be z=0 at the rear end, z=L at the front end and z=L1 at the starting point of the taper (0 less than L1 less than L). L is the length of the LD. (Lxe2x88x92L1) is the length of a tapering portion 15 of the active layer 14. The top-tapered SSC-LD gives the stripe the width w(z) and the thickness d(z) as functions of z,
0xe2x89xa6zxe2x89xa6L w(z)=wo,xe2x80x83xe2x80x83(1) 
0xe2x89xa6zxe2x89xa6L1 d(z)=do,xe2x80x83xe2x80x83(2) 
L1xe2x89xa6zxe2x89xa6L d(z)=doxe2x88x92xcex1(zxe2x88x92L1),xe2x80x83xe2x80x83(3) 
xcex1 greater than 0, doxe2x88x92xcex1(Lxe2x88x92L1) greater than 0.xe2x80x83xe2x80x83(4) 
Here, a is an inclination angle of the tapered end of the stripe. Inequality (4) signifies a definite thickness at the front end (z=L). The narrowing active layer cannot enclose the light completely and allows the light to leak from the active layer. The leak of the light power out of the stripe layer increases the beam diameter. The (beam) spot size is enlarged by the tapered stripe. Since the sectional area is enlarged, the dispersion angle of the beam is reduced to the contrary. The beam spot size is inversely proportional to the beam dispersion angle. The reduction of the dispersion angle is important for the SSC-LDs.
FIG. 4 shows a horizontally-tapered SSC-LD. The thickness d of the active layer is constant. The breadth w is narrowed toward the front end. A coordinate is defined to be z=0 at the rear end, z=L at the front end and z=L2 at the starting point of the taper (O less than L2 less than L). L is the length of the LD. (Lxe2x88x92L2) is the length of a tapering portion 16 of the active layer 14. In the horizontally-tapered SSC-LD, the stripe 14 has the width w(z) and the thickness d(z) as functions of z,
0xe2x89xa6zxe2x89xa6L d(z)=do,xe2x80x83xe2x80x83(5) 
0xe2x89xa6zxe2x89xa6L2 w(z)=wo,xe2x80x83xe2x80x83(6) 
L2xe2x89xa6zxe2x89xa6L w(z)=woxe2x88x92xcex2(zxe2x88x92L2),xe2x80x83xe2x80x83(7) 
xcex2 greater than 0, woxe2x88x92xcex2(Lxe2x88x92L2) greater than 0.xe2x80x83xe2x80x83(8) 
Here, xcex2 is a reduction angle of the tapered sides of the stripe. Inequality (8) signifies a definite width at the front end (z=L). The narrowing active layer induces insufficient enclosure of the light. The leak of the light from the active layer enlarges the beam spot size. The increase of the spot size reduces the beam spreading angle. Both the SSC-LDs (FIG. 3 and FIG. 4) have a function of reducing the beam spreading angle.
The SSC-LD is not an entirely novel device but is not well known for the skilled in art yet. The fact requires some description of the SSC-LD. Conventional LDs emit small size spot beams with large dispersion angles. A lens is indispensable for the prior LD modules with the conventional LDs. The use of the lens, however, raises the part cost, the assembling cost and the package cost. Elimination of the lens enables the LD module to reduce the cost. The purpose of contriving the SSC-LD is to eliminate the lens form LD devices.
It is desired to combine an optical fiber to an LD without lens. Small dispersion of the LD beam seems to be sufficient for the non-lens coupling between the fiber and the LD. In fact, the LD stripe has a small breadth (1.21 xcexcm) and a small thickness (0.21 xcexcm). Someone may suppose that the direct touching of the fiber to the LD would introduce the LD light into the fiber with high efficiency. But it is not true. The excess size difference between the fiber and the LD impedes the introduction of the LD beam into the fiber. If the beam size of the laser is enlarged up to the size of the fiber core, the coupling efficiency will be raised.
If the waveguide of the LD is narrowed, the power of localizing the light power within the waveguide is weakened. The decline of localizing the light power into the waveguide core allows the light power to pervade outside of the waveguide of the LD stripe. The diffused light power will enhance the coupling efficiency between the LD and the fiber. The SSC-LD with the narrowed stripe (waveguide) aims at enhancing the coupling efficiency by enlarging the LD beam spot size near the fiber core size through the narrowed stripe. Thus, such a laser diode is called a spot-size converted laser diode (SSC-LD). An increase of the spot size (beam diameter) will invite a decrease of the aperture of the beam. The narrow aperture will enable the LD to introduce the light power into the fiber with high efficiency without converging lens.
The SSC-LD with the narrow stripe end was invented from the idea of decreasing the aperture angle by increasing the beam (spot) size through the narrowness of the stripe. The end of the active stripe is sharpened for decreasing the aperture angle of the emission beam in the improved LD as shown in FIG. 3 and FIG. 4. Various contrivances have been suggested for improving the SSC-LDs.
{circle around (1)} Japanese Patent Laying Open No.9-61652, xe2x80x9cA semiconductor waveguide and a method of making samexe2x80x9d, proposes a tapered waveguide and an improved LD having a tapered waveguide as an active stripe. The single tapered waveguide can convert the spot size by coupling to a conventional laser diode (LD).
{circle around (2)} Japanese Patent Laying Open No.11-220220, xe2x80x9cA semiconductor laser and a method of making samexe2x80x9d, suggests an LD having an active layer with a narrowing width, as shown in FIG. 4. The narrowing angle xcex2 of inequality (8) is less than 0.14 degree. The narrowing width enables the LD to generate a narrow aperture beam.
{circle around (3)} Yoshio Itaya, Toshihiko Sugie, Mitsuo Yamamoto, xe2x80x9cSpot-size converted lasers (SSC-LD)xe2x80x9d, Technical Report of IEICE (The Institute of Electronics, Information and Communication Engineers), OPE95-140, LQE95-134(1996-2), p31, makes SSC-LDs on a two-inch InP wafer. The SSC-LD has a 600 xcexcm length and a 300 xcexcm width. The active layer (emission stripe) is gradually reduced from 1.5 xcexcm to 0.3 xcexcm. The SSC-LD is coupled to a light waveguide without lens. The coupling loss is 1.2 dB to 2.5 dB. The coupling loss is sufficiently small.
{circle around (4)} Y. Inaba, M. Kito, M. Ishino, T. Chino, T. Nishikawa, T. Uno, Y. Matsui, xe2x80x9c1.3 xcexcm tapered-active-stripe laser with low threshold and high slope efficiencyxe2x80x9d, Technical Report of IEICE (The Institute of Electronics, Information and Communication Engineers), EMD97-43, CPM97-81, OPE97-59, LQE97-55(1997-08), p81, proposes SSC-LDs having a active layer with a narrowing width. The tapering width of the active layer is 0.61 xcexcm at the front end and 1.6 xcexcmxcx9c2.6 xcexcm at the rear end. Various SSC-LDs having a variety of tapering angles are made. The SSC-LDs are coupled to a fiber without lens. The coupling efficiency is measured by changing the distance between the SSC-LD and the fiber end. The maximum coupling efficiency is xe2x88x924.7 dB at an optimum distance. The range of xc2x13 xcexcm from the optimum distance obtains coupling efficiency of higher than xe2x88x924.7 dB. The coupling efficiency of the conventional LDs to fibers is xe2x88x929 dB at the highest. Thus the SSC-LD obtains an extra increase of 4 dB of coupling efficiency in comparison to the conventional LDs.
{circle around (5)} Yoshio Itaya, Osaake Nakajima, Mitsuru Naganuma, Mitsuo Fukuda, Kiyoyuki Yokoyama, Hiromu Toba, xe2x80x9cOptical Semiconductor Devices for Hybrid Modulesxe2x80x9d, NTT RandD, vol.46, No.5, 1997, p487-490, proposes a spot-size converter laser diode (SSC-LD) having a tapering active stripe layer with a reducing thickness end. The thickness of the active layer is 0.3 xcexcm in the oscillating region but is reduced to 0.1 xcexcm at the front end. The active layer has a constant width of 1.5 xcexcm. The half width angle of the emission aperture of the laser is 6 degrees to 9 degrees. The full aperture angle is 12 degrees to 18 degrees. In the conventional laser diodes with a constant stripe, the beam aperture is 30 degrees to 40 degrees. The SSC-LD succeeds in confining the LD beam within about a half angle of the conventional LD aperture.
{circle around (6)} xe2x80x9cLight/microwave semiconductor application technologyxe2x80x9d, Science Forum Corporation published Feb. 29, 1996, first print, first copy, p165, xe2x80x9cDigital Communication Devicesxe2x80x9d, takes a review of the spot-size conversion laser diodes. The report says that the conventional LDs emit wide beams with an aperture angle of 33 degrees but the SSC-LDs make narrow beams of a 9 degree aperture angle. The review writes that the loss of the non-lens coupling is about 10 dB for the conventional LDs and about 4 dB for the SSC-LDs.
{circle around (7)} Japanese Patent Laying Open No.2-195309, xe2x80x9cPhotocoupling devicexe2x80x9d, proposes a passive device with the spot-size conversion function. The device has a substrate with width-varying waveguides having the spot-size conversion function. Instead of coupling a fiber to another fiber with a lens, the invention employs the proposed SSC-waveguide for coupling a fiber to another fiber. An increase of the core width w results in a reduction of the beam size (spot size) in the waveguide. A decrease of the core width w invites an increment of the beam size. The fibers are coupled by the light waveguide having a narrow core intermediate part and wide core ends. The wide core ends enhance the coupling efficiency through the reduction of the beam sections at both ends.
{circle around (8)} Nobuaki Hiraguri, Kazuo Shiraishi, xe2x80x9cCoupling characteristics between integration-oriented lensed fibers and LD""s with narrow beam divergencexe2x80x9d, Technical Report of IEICE (The Institute of Electronics, Information and Communication Engineers), EMD97-44, CPM97-82, OPE97-60, LQE97-56(1997-08), p87, proposes a ball-lensed fiber coupling to the SSC-LD (LD""s with narrow beam divergence) with high coupling efficiency. The fiber is called a xe2x80x9cball-lensed fiberxe2x80x9d since it has an end being shaped in a ball lens. A fiber is ball-lensed by melting an end of the fiber by oxygen-hydrogen flame. The molten glass makes a ball at the end by the surface tension. The balled end is cooled into a ball lens. This report employs and compares coreless lensed fibers and GI lensed fibers.
The coreless fiber means a fiber without the core-cladding structure. The core-cladding structure is rather an obstacle when the fiber is changed to be a lens at the end. Thus, the special convenient coreless fiber is employed. An object fiber is a conventional single mode fiber having a core of a 10 xcexcm diameter and a cladding of a 125 xcexcm diameter enclosing the core. The coreless fiber of a 125 xcexcm diameter is made of the same material as the cladding of the single mode fiber. A 1 mm long coreless fiber is fused with a single mode fiber. The coreless end is heated by the oxygen-hydrogen flame into a ball lens.
The GI fiber is a graded-index fiber having a core of continually decreasing refractive index in the radial direction down to the refractive index of the cladding at the boundary. Both the ball-lensed coreless fiber and the ball-lensed GI fiber can be coupled to the SSC-LD with small coupling loss.
However, the coupling by the ball-lensed fibers has some drawbacks. It is difficult to fuse a short, tiny fiber to an end of a narrow single-mode fiber. The end is melted into a ball lens by the oxygen-hydrogen flame. The process depends upon an accident. The diameter of the ball lenses is a stochastic variable. It is difficult to make ball lensed fibers with the predetermined size. The tolerance of the positioning of the end is excessively narrow. It takes much time to align the fiber and the laser diode. The device consisting of the ball lensed fiber and the SSC-LD is a sophisticated device but is not suitable for mass-production. It will be an expensive device. This device aims only at the direct coupling of the fiber to the SSC-LD. The proposal makes the ball lens at the end of the fiber for improving the direct coupling.
The prior art of the SSC-LDs has been described. This invention can be applied to any kinds of the SSC-LDs.
The direct coupling to a fiber is the sole purpose of the SSC-LDs. Thus, the prior SSC-LDs have been stuck either directly to the end of a light waveguide as shown in FIG. 5 or directly to the end of an optical fiber as shown in FIG. 6.
FIG. 5 is a plan view of a horizontally-tapering SSC-LD 18 and a light waveguide 20. The SSC-LD 18 has an active layer 14 having a horizontally narrowing part 16. The light waveguide 20 has a core 21 with a higher refractive index than the cladding parts. The core 21 is coupled to the tapering front end 16 of the SSC-LD 18. The SSC-LD 18 is directly joined to the waveguide 20 without lens. The light made in the parallel active layer 14 by the injected current is enlarged in the tapering portion 16 in spot size but is diminished in aperture angle. The narrow aperture angle allows the LD light to enter the core 21 of the light waveguide 20 with high efficiency.
FIG. 6 is a vertically sectioned view of a vertically-tapering SSC-LD 19 and an optical fiber 22. The SSC-LD 19 has an active layer 14 having a vertically narrowing part 15. The fiber 22 has a core 23 with a higher refractive index than the cladding parts. The core 23 is coupled to the tapering front end 15 of the SSC-LD 19. The SSC-LD 19 is directly joined to the fiber 22 without lens. The light made in the parallel active layer 14 by the injected current is enlarged in spot size but is diminished in aperture angle in the tapering portion 15. The narrow aperture angle allows the LD light to enter the core 23 of the fiber 22 with high efficiency.
The SSC-LDs have been contrived for coupling to a fiber or a waveguide with a small NA (Numerical Aperture) with high coupling efficiency. The SSC-LD dispenses with a lens for coupling with the fiber or the waveguide. The small aperture angel prevents the LD beam from dispersing out of the waveguide or the fiber. The omission of lens is the most important object of the SSC-LDs. The skilled believes there is no room for a lens in the coupling between the SSC-LD and the fiber/waveguide. xe2x80x9cNon-lens couplingxe2x80x9d is the most conspicuous character of the SSC-LDs.
On the contrary, the conventional LD module as shown in FIG. 1 has an LD which emits a wide aperture angle (30 degrees to 40 degrees) beam. The converging lens 9 is indispensable in the LD module. The lens 9 is a ball lens, a spherical lens or an aspherical lens which is chosen in accordance with the purpose or performance.
In the conventional LD modules (transmission modules), the choice of a ball lens or an aspherical lens depends upon the low-power/low-cost or the high-power/high-cost. Namely, low-cost and low-power LD modules have cheap ball lenses. High-cost/high-power LD modules are equipped with sophisticated aspherical lenses in the prior optoelectronic devices. One purpose of the present invention is to provide a low-cost ball-lens LD module which can generate light power as high as the conventional high-cost aspherical lens LD modules. Another purpose of the present invention is to provide an aspherical lens LD module which can generate stronger light power than the conventional high-cost aspherical lens LD modules.
The present invention proposes an LD module having a spot-size conversion LD with a narrow beam aperture angle (of 10 degrees to 20 degrees), a light guide medium (a fiber or a waveguide) and a lens for converging the LD light to an end of the light guide medium. The novelty lies in the coupling of an SSC-LD and a lens.
The spot-size conversion laser diodes have been known to the skilled, as described hitherto. The module of a conventional LD and a lens shown in FIG. 1 is also known to the skilled. The present invention proposes a new coupling of an SSC-LD and a lens which will be obtained from FIG. 1 by replacing the conventional LD by the SSC-LD. In short, the present invention suggests an assembly of (SSC-LD+lens) instead of (LD+lens).
The SSC-LDs have been developed and investigated for eliminating the converging lens. The skilled in art believes that there is no probability of interposing a lens between the SSC-LD and the fiber. In any cases, the SSC-LDs have been directly coupled to the fibers or the waveguides.
The Inventors hit an idea of applying positively the converging optics (lens) to the spot-size conversion laser diode which can do without lens.
Instead of joining the SSC-LD directly to the medium (fiber/waveguide), this invention tries to couple the SSC-LD via a lens to the medium (fiber/waveguide). Namely, the module of the present invention is an air-gaped coupling system consisting of an SSC-LD, a lens and a medium which are separated by air gap.
The aperture angle of the beam emitted from the SSC-LD is so narrow that even a ball lens is nearly free from spherical aberration. The SSC-LD enables a cheap ball lens to converge almost all the LD rays. The present invention can introduce higher power into the medium than the conventional LD module as shown in FIG. 1. This is an important feature which will be later described in detail.
The rate of the LD spot size to the fiber/waveguide core is reduced to about a half. The length of the module can be shortened.
If a cap having a lens as shown in FIG. 1 is employed, the LD module is completely hermetically sealed, which raises the reliability of the LD module. The metal package which is a standardized inexpensive metal case can be used. Thus, the present invention can give a low-cost, high-power LD module.
The advantages of the present invention are described here. The present invention succeeds in raising the LD/medium coupling efficiency by 50% to 100% by employing the SSC-LD emitting a beam of a wide spot size and a narrow aperture angle.
Lower injection current produces the same light power as the conventional module of FIG. 1. The present invention saves the current and alleviates the electric power consumption. The low driving current prolongs the lifetime of the laser and enhances the reliability of the module.
Inherently, the SSC-LDs have been developed for the purpose of removing the converging lens and directly coupling the medium to the LD. Any persons have believed in the lensless, direct coupling of the SSC-LD to the medium. The Inventors have first noticed the excellent indirect lens-used coupling of the SSC-LD to the medium.